基于实验室模拟研究二次有机气溶胶生成及老化机制

由于二次有机气溶胶(SOA)对气候变化、区域污染和人体健康具有明显的影响, 因此受到了广泛的关注. 基于实 验室的方法可以在稳定可控的条件下探讨 SOA 的生成机制, 其中环境烟雾箱和气溶胶生成潜势反应器是最常用的两 种模拟工具. 本文综述了基于这两种模拟工具对 SOA 产率的影响因素、SOA 生成机制和 SOA 老化过程中性质演变特 征的研究. 影响 SOA 产率的因素主要包括 OH 暴露量, NOx水平(VOCs/NOx), 种子颗粒物的浓度及种子颗粒物的化学 组成. SOA 产率随着 OH 暴露量、VOCs/NOx 比值的增加均先增后减; 种子颗粒物的存在会通过提高气态中间产物的凝 结汇, 从而促进物质从气相到颗粒相的转化; 酸性种子颗粒物可以通过提高摄取系数并提供酸催化条件促进 SOA 的 生成; 种子颗粒物中的金属离子和矿物质也会通过催化作用或者影响氧化剂的产生等过程对 SOA 的生成和老化产生 作用. 本综述还总结了不同源排放气态前体物SOA的生成潜势以及生成SOA的特征. 等效光氧化龄约为2～3 d时, 汽 油发动机排放生成 SOA 量达到最高值, 增长倍数(SOA/POA, POA 即为一次有机气溶胶)约为 10～14, SOA 生成潜势约 为 400～500 mg/kg fuel; 生物质燃烧排放, 在等效光氧化龄约为 3～4 d 时, SOA 增长倍数最大, 平均约为 1.42～7.6; 而 其他源如天然气燃烧、餐饮等排放也具有很高的 SOA 生成潜势, 天然气燃烧排放后 SOA 的增长倍数高达 268 倍, 餐 饮源排放 SOA 的增长倍数约为 3～8 倍. 不同地区的实际大气中气态前体物氧化生成 SOA的最高值出现在等效光化学 龄为 2～4 d 时. 综合不同研究中源排放和实际大气中前体物生成 SOA 演化特征发现, 随着 OH 暴露量的增加, SOA 的 氧化态逐渐增加, O/C 比约从 0.2 增长到 1.3, O/C 与 H/C 变化斜率均在－1 到 0 之间, 说明氧化机制可能包括生成羟基、 过氧羟基以及羧酸基团的物质; 氧化过程中 SOA 的挥发性逐渐降低, 吸湿性逐渐增加. SOA 生成过程中间态物种的测 量技术开发、复杂体系下 SOA 生成机制的研究和 SOA 演化过程中特征的表征是未来 SOA 研究的重要方面. 

Secondary organic aerosol (SOA) is a major component of aerosols in the atmosphere, which plays a crucial role in climate change, regional pollution and human health. Laboratory simulations are usually used to mimic SOA formation. The most commonly used simulation facilities are environmental chambers and potential aerosol mass (PAM) reactors. Here in this work, we review the studies about influencing factors and mechanisms of SOA formation, as well as the evolution of SOA aging. We summarize the influencing factors on SOA yields, i.e. OH exposure, NO_x level, and the loading and chemical composition of seed particles. The effects of NO_x level (i.e. VOCs/NO_x) and OH exposure are nonmonotonic. The NO_x level influences the fate of RO₂ radicals, so SOA yields will increase and then decrease with the addition of NO_x. Similarly, the increase of OH exposure affects the major oxidation mechanism from functionalization to fragmentation, leading to the up and down trend of SOA yields. The higher seed particle loading provides more surface area for condensable products and then increases the SOA yields. The particle acidity favors the uptake process for gas-phase products and promote the SOA formation via further reactions in the condense phase. Trace components e.g. transition metals and minerals can be involved in the SOA formation and aging by catalysis or affecting the uptake of oxidants and their products. Chambers and PAM reactors are usually used to explore SOA formation potential of different sources. SOA formation potential from vehicles will be influenced by engine types, engine loading and composition of fuel. The highest SOA enhancement ratio (SOA/POA) from gasoline engines is about 4~14, when the equivalent photochemical days are 2~3 d. The SOA production mass from gasoline vehicles is from about 10~40 to 400~500 mg/kg fuel. The SOA formation potential is about 400~500 mg/kg fuel. The largest SOA enhancement ratio for biomass burning is 1.4~7.6, which occurs at 3~4 photochemical days. The SOA enhancement ratio from ambient air differs from region to region. However, the highest ratios all occur at the photochemical age of about 2~4 d. We summarize the SOA characteristics evolution with aging. Oxidation state of particles will increase with OH exposure. Changes of H/C and O/C with increasing OH exposure can be plotted in the Van Krevelen diagrams. The slopes of fitted curve range from -1 to 0, indicating OA evolution chemistry involving addition of carboxylic acids or addition of alcohols/peroxides. In addition, the volatility and hygroscopicity of oxidized OA will be higher than primary organic aerosols. In the future, more studies should be focused on developing new technologies to measuring the oxidized intermediate products at a molecular level. Also the researches on the mechanism of SOA formation from complex precursors are also crucial to understand the SOA formation at real atmosphere.